For watch cover glasses, blue plate glass (soda glass), white plate glass, sapphire glass and the like are used. These cover glasses have a high reflectance in the visible light region and have problems on visibility for watch hands and dials. That is to say, when we check time by a watch under various environments, for example, indoor, outdoor, day or night, there is a problem such that it is difficult for us to look the time indication of the watch because outdoor daylight or illumination changes and thereby the surface of a cover glass of the watch reflects these lights.
In order to solve the above problems, it is well known that the both surfaces or at least one surface are coated with an antireflection film (for example, referred to Patent document 1).
Antireflection films are generally prepared by making and combining an oxide film, a nitride film, a fluoride film or a sulfide film of a metal or an inorganic substance having an appropriate refractive index into a film having a desired reflectance in a limited wavelength region.
However, the antireflection film disclosed in Patent document 1 has inferior flaw resistance etc. Patent document 1 discloses a wristwatch equipped with a cover glass, which is coated with an antireflection film having an outermost layer of magnesium fluoride. When the wristwatch is used for a long period of time, it has problems such that the surface of the antireflection film is minutely flawed or peeled off, and thereby the surface is fogged and the watch hands or dials are hardly visible. Furthermore, even if SiO2 having a film hardness higher than that of magnesium fluoride is used, the above problems cannot be solved.
Patent document 2 is known as a conventional technique of solving the above problems disclosed in Patent document 1.
Patent document 2 discloses the technique that the film hardness is improved by adding a slight amount of nitrogen to a SiO2 material constituting an antireflection film provided on a cover glass.
In the antireflection film disclosed in Patent document 2, there is still room for improvement on effective hardness and flaw resistance.    Patent document 1: JP-A-2002-202401 (p. 2, FIG. 1 and FIG. 2)    Patent document 2: JP-A-2005-114649 (pp. 3 and 4, FIG. 1)